Silicon Phase Control And Property Of The Al-50wt.%Si Alloy

Ultra-high silicon aluminum alloy is widely used in electronic packaging field due to the combination of excellent properties, such as low density, low coefficient of thermal expansion, high thermal conductivity. In this study, ultra-high silicon aluminum alloy(wt%si=50%) was chosen as the research object, the effects of the superheat temperature, the weight percent of refiner and the spray deposition technique on the morphology and size of the primary silicon were investigated. Moreover, the variation mechanism of the primary silicon at different conditions were studied. In addition, the variation of the mechanical property, the thermal expansion behaviour as well as the thermal conductivity of the alloy were also studied.The primary silicon mainly displays plate-like morphology in as-cast alloy, the size decreases from 171μm to 103μm with superheat temperature increasing from 100℃ to 500℃. However, its morphology did not show significant change. The addition of refiner P promotes the formation of fine particulate Al P phase, which can provide heterogeneous nucleation sites for primary silicon, the morphology of primary silicon changes from plate-like shape to polygon gradually. The average size of primary silicon reaches 38μm with 0.5wt.%P addition. Whereas the Al P phase will segregate with an excess of P addition, the morphology of Al P phase varies from particulate to needle-like shape, which leads to the decreased nucleation sites for primary silicon phase, and the average size of primary silicon increased. Moreover, the addition of P changes the nucleation temperature of the primary silicon phase. Due to the presence of an excess of P(~1.3wt.%), the Al P phase nucleates at a lower degree of supercooling, the Si atoms in the melt nucleate around Al P phase, thus increases the nucleation temperature of primary silicon(~22.6℃), however, a higher degree of supercooling is needed with 0.5wt%P, the P atoms have negative effects on the nucleation of Si phase, which leads to the decrease of the nucleation temperature of primary silicon phase(~55℃). Due to the increase of the cooling rate, the primary Si is further refined with an average size of 11.6μm in the spray deposited alloy. The effect of the addition of P on primary silicon is not obvious when solidified at high cooling rate.The growth mechanism of the primary silicon fabricated at different conditions were studied by etching with mixed acid extraction. The results show that plate-like primary silicon mainly grows with twin plane reentrant edges(TPRE) mechanism. The addition of P changes the morphology and the size of primary silicon, however, the growth mechanism of primary silicon is still TPRE, the primary silicon grows into a reunion shape. The primary silicon that nucleates and grows spontaneously is mainly characterized by plate-like shape with low aspect ratio. With increased cooling rate, the growth pattern of the primary silicon transforms from facet to mixture of facet and non-facet. The primary silicon grows with a strong anisotropy at low cooling rate, and the anisotropy trend weekened with increased cooling rate. Therefore, a large number of bulk polyhedron primary silicon presents when prepared by spray deposition.The grain size of Al-50 wt.%Si alloy is much lower when prepared with spray deposition than the as-cast alloy, the size of primary Si is also significantly reduced. The deformation activation energy of the alloy is 413.17 k J/mol, and the deformation is inhibited severely due to the high content of Si. The hot processing maps are established, and the deposited alloy exhibits unstable region with a low temperature and a low strain rate, the maximum power dissipation η is acquired at high temperature. Molding pressing and hot isostatic pressing(HIP) technologies are used for densification of the alloy.The results show that the molding pressing technology is suitable for densification of the alloy after P refinement. The eutectic silicon transforms from long rod to sphaerolitic shape during the densification and solution. The hot isostatic pressing is suitable for densification of the deposited alloy, the sharp corners become passivation in the deposited alloy after densification. The ultimate tensile strength(UTS) of the alloy without P addition after heat treatment is only 36.7MPa, and the fracture surfaces exhibit obvious transgranular fracture. However, the UTS of the alloy with P refiner can reach 160.3MPa accompanied with a small amount of dimple fracture, which shows an increase about 336.8% compared with non-refined alloys. The increase of the UTS can be ascribed to the refinement of primary silicon and the reduction of the defects inside the primary silicon. The size of the primary silicon is further decreased after spray deposition, the UTS is 167.8MPa after hot isostatic pressing and annealing, and the alloy presents intergranular fracture morphology.The coefficient of thermal expansion(CTE) of the Al-50 wt.%Si alloy fabricated by traditional casting technology decreases as a result of increased porosity and refinement of primary Si. But for the deposited alloy, part Al dissolves into primary silicon due to rapid cooling, which increases the CTE and the CTE is higher than that of the as-cast alloy. During the densification process, part of the dissolved Al atoms precipitates from the primary silicon, and the CTE of the alloy decreases. The larger size of primary silicon in the alloy without P refiner would separate the Al matrix, and seriously impede the electron transfer of Al matrix, the thermal conductivity(TC) of the alloy is lower than the alloy with P refiner. The presence of pores also reduce the TC of the alloy, the TC of the alloy increases after densification. After HIP, the decreased extent of TC of the spray deposited alloy is lower than the alloy with P refiner. The TC of the HIP alloy is lower than P refined alloy at low temperature, but with increased temperature to 200℃, the thermal conductivity of the alloy is higher than the alloy with P refiner. The CTE and TC of both P refined alloy and spray deposited alloy can satisfy the requirement of the thermal performance of electronic packaging materials(CTE: 7~13×10-6/K, TC>100W/m·K). Compared with other preparation methods, alloys prepared in our study possess more excellent comprehensive performance.